The invention relates to a device for monitoring the air pressure of a tire in motor vehicles using a sensor and an evaluation unit. The sensor delivers a signal corresponding to the deformation of the tire. The evaluation unit receives the signal from the sensor.
A device of this kind for monitoring tire air pressure is known, for example, from German Patent document DE-39 16 176 A1. In the known device, a space-measuring device is mounted as an evaluation unit on the rim of a vehicle tire. This space-measuring device contains a transmitter which emits laser beams into the interior of the tire tread rubber (which acts as a diffuser-reflector). The laser beams reflected from the interior of the tire are received by a receiver that, in this case, functions as a sensor to deliver a signal that corresponds to the deformation of the tire. The deviation of the directions of the reflected rays from one another are evaluated by the space-measuring device as a measured value indicating the degree of radial tire indentation, which depends upon the tire air pressure.
A device for monitoring tire air pressure is known from EP 0 197 813 B1, in which the indentation of the tire is monitored from the reflection of emitted sound waves.
It is a disadvantage in the known devices for monitoring tire air pressure in motor vehicles, that the determination of the deformation of the tire using radiation or sound waves is extremely awkward and expensive.
There is therefore needed to provide a device for monitoring tire air pressure in motor vehicles, in which, firstly, a sensor of simple design is used and, secondly, a simple but accurate evaluation of the deformation of the tire is possible.
The present invention meets these needs by providing a device for monitoring tire air pressure in motor vehicles using a sensor and an evaluation unit. The sensor delivers a signal corresponding to the deformation of the tire. The evaluating unit detects the signal from the sensor. The sensor is designed to deliver an electrical pulse corresponding to the degree of deformation of a tire during a footprint pass. A sensor, especially a bending or extension sensor for measuring the deformation of a tire during a footprint pass is designed so that it delivers an electrical pulse corresponding to the degree of this deformation. Preferably the deformation of the tire sidewall is measured during the footprint pass. The "footprint" is of the tire on the road, at the point where the tire increasingly flattens out as tire pressure falls. The pressure is measured every time the point on the tire where the sensor is located comes around to form the footprint, hence a "footprint pass".
The evaluation of an electrical pulse, for example by measuring its amplitude, its duration, or the frequency with which this pulse repeatedly appears, requires a much lower expenditure for the evaluation unit than the evaluation of a reflected optical beam or sound waves. These must first be processed to produce an electrical signal that can be evaluated. Accordingly, the device according to the present invention makes it possible to detect the deformation of a tire simpler and faster.
In an advantageous embodiment of the present invention, the evaluating unit evaluates only the height of the amplitude of the electrical pulse, in order to determine the degree of deformation. This is an especially simple way to determine the deformation of the tire when high accuracy is not a requirement. For example, the influence of vehicle speed on the deformation of the tire is disregarded in this case.
In another advantageous embodiment of the invention, the evaluating unit determines the ratio of the duration of the electrical pulse to the duration of one revolution of the wheel, and determines the degree of deformation from this ratio. This is a more precise approach to determining the deformation of the tire, for example, as a function of vehicle speed as well.
Another advantageous embodiment of the present invention delivers only one electrical pulse per revolution of the wheel. Accordingly, only one sensor need be provided per wheel.
Yet another advantageous embodiment of the present invention detects the number of pulses over the entire service life of a tire. For example, a counter can be provided in the tire or on the evaluating unit. The counter can be reset only when a tire is changed. By recording the number of pulses over the entire service life of a tire, its performance can be computed and its wear estimated. This is helpful for example when deciding whether to continue using a tire or to re-use it.
Another advantageous embodiment of the invention has the sensor mounted directly on or in a zone of the tire that is deformed during a footprint pass, advantageously with maximum force. For example, it can be mounted on or in a sidewall of the tire. This direct connection between the sensor and the measurement location permits exact determination of the deformation at this measurement location.
Another advantageous embodiment of the invention preferably makes use of a piezo element as a sensor. This is an especially simple sensing device to passively determine the bending event resulting from tire deformation.
Still yet another advantageous embodiment of the present invention transmits the electrical pulse delivered by the sensor without wires to the evaluating unit. Because of the fixed location of the evaluating unit outside the tire, only the sensor and a transmitter for wireless transmission need be provided in the tire itself. The susceptibility of the evaluating unit to interference is reduced by its fixed location outside the tire, and only the least expensive components are integrated into the tire.
In another advantageous embodiment of the invention, the sensor is connected directly to a transmitter for wireless transmission of the pulse. The sensor and the transmitter are therefore mountable as an integrated component on or in the tire, thereby taking up only a little space.
In yet another advantageous embodiment of the invention, the piezo element is used not only as a sensor but also to supply electricity to the transmitter. For example, an electrical battery can be connected with the piezo element, and charged by the electrons released by the piezo element during every bending event or after a certain number of bending events.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.